Pregnant guinea pigs are injected subcutaneously daily with 2.5 Mu g of diethylstilbestrol (DES) (increased to 5.0 Mu g later in pregnancy) or with 50 Mu g of Beta-estradiol (E) in sesame oil from about the 45th day to term (62 days). A second group receives 50 Mu g of one hormone or the other daily for 3 days after birth. The animals are allowed to mature and are studied for: (1) cycling activity, onset and regularity, (2) evidence of pituitary-ovarian imbalance (unregulated estrogen stimulation of vagina, cervix, uterus and tubes; ovarian atresia), (3) levels of estrogen in the blood by radioimmune assay. Particular attention is paid to the occurrence of metaplastic changes in the uterine epithelium which may represent pre-malignant conditions.